1. Field of the Invention
The invention relates to an image forming apparatus which reads an original document by using a photoelectric conversion element and forms an image.
2. Description of the Related Art
An image forming apparatus is provided with an image reading device to read an original document image by using a CCD sensor. This image reading device corrects high-frequency distortion in an image signal, low-frequency distortion, and distortion of light-emitting characteristic. The high-frequency distortion is the distortion caused by variations in sensitivity of photoelectric conversion sensors corresponding to respective pixels constituting the CCD sensor. The low-frequency distortion is the distortion caused by an optical system to guide light from the original document to the CCD sensor. The distortion of light-emitting characteristic is the distortion (uneven light emission) of the light irradiated to the original document from a light source. In order to correct these distortions and unevenness, in the image forming and reading apparatus, a shading correction is generally performed on an output signal of the CCD sensor.
In this shading correction, an image signal is corrected using a black reference signal which is black reference and a white reference signal which is white reference. Especially, the white reference signal is acquired by reading an image of a white reference member by a CCD sensor (JP-A-9-294207). Thus, in the case where foreign matter such as a flaw or a contamination is attached to a part of the white reference member, an image of the foreign matter is read as the white reference. In this case, at the position corresponding to the foreign matter, the white reference signal has a value different from a desired value. As a result, a white streak-like image can appear on a read image of an original document.